Non-linear crystals for second harmonic generation (SHG) or other frequency conversion devices, such as Sum Frequency Generation (SFG) or Third Harmonic Generation crystals, are highly polarization sensitive converting only one linearly polarized light (horizontal or vertical depending on the crystal cut). Modules providing linearly polarized pump light coupled by polarization maintaining (PM) optical fiber to the non-linear crystal have been constructed to satisfy the polarization sensitivity of the non-linear crystal. However, such an optical system produces power fluctuation and noise in its output.
Second harmonic generation is a commonly practiced technique for obtaining coherent light at short wavelengths from long wavelength laser sources. It is a non-linear process where an optical beam, called the pump beam, interacts with an optically non-linear medium, in the case of second harmonic generation, to generate a second harmonic beam, where the frequency of the second harmonic beam is twice the frequency of the pump beam. Equivalently, the free space wavelength of the second harmonic is half the free space wavelength of the pump. Any material which lacks inversion symmetry can be used as the optically non-linear medium for second harmonic generation. Materials which are commonly used include lithium niobate, MgO-doped lithium niobate and KTP (KTiOPO4). Second-harmonic generation is one of a class of methods, known collectively as non-linear frequency mixing. A typical waveguide device incorporating a non-linear crystal uses a periodically poled ridge waveguide optical structure to generate or amplify coherent light at a desired wavelength from light at an input, or from a pump.
A SHG module 100 is illustrated at FIG. 1 including an external cavity laser 10, a PM fiber optical link 20, and a non-linear crystal waveguide structure 30. The external cavity laser 10 comprises a semiconductor gain chip 12 provided in a hermetic package 14 having a hermetic fiber coupling to a PM fiber external cavity 16 having a fiber grating 18. The PM fiber optical link 20 comprising the external cavity 16 and the fiber link 22 to the non-linear crystal waveguide structure 30, can be coupled into the grating 18 in discreet segments fusion coupled into either end of the grating 18, or a fiber Bragg grating can be imprinted directly into a single fiber link from the semiconductor gain chip 12 to the non-linear crystal waveguide structure 30. The non-linear crystal waveguide structure 30 is also provided within a hermetic package 32 having a hermetic fiber coupling to the fiber link 22 and a further hermetic fiber coupling to an output fiber 34. A collimator 36 terminates the output fiber 34 for coupling into an optical system. As stated above however, such an optical system produces power fluctuation and an additional excess noise in its output.
Close analysis of this problem has revealed the factors contributing to instability of the harmonic frequency conversion module power output and noise. PM fibers are used for transmitting polarized light. But their transmission is high for both polarizations, therefore polarization quality of the output light depends on: a) polarization quality of incoming light, b) quality of alignment of input and all intermediate elements. If the incoming polarization is easy to control, quality of alignment is often limited. If it is non-ideal, inevitably two orthogonal polarization modes with different propagation speeds are created in the PM fiber.
Since PM fiber is temperature sensitive, with changes in temperature causing the birefringence, the Δn difference in refractive index between the orthogonal axes, to vary, the two orthogonal polarization modes have phase shift at the output of the fiber which changes with fiber temperature. In addition, external stresses applied to the PM fiber, such as by solder at fiber ferrules, holders and feedthroughs cause changes to the polarization properties of the light. Therefore even ideally aligned polarization before soldering may be spoiled after soldering. The result of these effects is observable statistically, but not controllable. If an analyzing device on the output (for example, a non-linear crystal) is polarization sensitive, polarization beating caused by interference will create power fluctuations as a function of non-controlled, or restrictedly controlled parameters. These parameters include the temperature of every element within the fiber route, stress on the fiber, laser pump current and pump power and atmospheric pressure, which can all affect the polarization extinction ratio (PER) within the optical fiber link. Such power fluctuations include a fast component, which creates additional noise with a broad spectrum.
This PER degradation results in light of both linear polarization states coupling into the PM fiber link. Light coupled into the fast and slow axes will arrive with a phase difference. If there are at least two PER degradation points, a mix of polarizations on each axis can result, causing interference. The relative phase difference changes with temperature, altering the interference and changing the amount of light coupled to the harmonic frequency converter. The result is seen as amplitude fluctuation in the harmonic frequency converter output.
While it is possible to control manufacturing processes to improve alignment of the birefringent fiber axes to the linearly polarized pump light, stress on the birefringent fiber at solder locations and hermetic feedthroughs is more difficult to control. Looking at FIG. 1, major polarization distortion points, the regions with the strongest polarization distortions due to stress in the fiber fixture are labeled X1. The fiber grating is also a potential source of polarization distortions and it is labeled X2 as minor polarization distortion points. A second parasitic linear polarization orthogonal to the first linear polarization converted by the non-linear crystal waveguide is introduced into the PM fiber link at these distortion points. The birefringence of the PM fiber causes interference between light traveling in the fast and slow axes. With changing temperature, the birefringence in the fiber changes altering the transmission power of the first linear polarization. This partially explains the output power fluctuation. A further disruption to the output power is contributed by noise.
A US patent application No. 2005/0226278 by Xinhua Gu et al. has also observed the PER distortion points in PM fiber at the fiber ferrule, the fiber holder or at a fusion splice. In that case, the authors are proposing a fiber laser for outputting high power short laser pulses. Their structure includes a mode locked fiber oscillator, a variable attenuator, an amplifier and a compressor for compressing the pulse width. They propose using waveplates and polarizers in the modules. “The linear polarizers counter the superposition of the phase shift from each polarization degrading element . . . by embedding linear polarizers throughout the series of modules, the PER of the aggregate system can be substantially controlled such that the intensity fluctuation is below about 1 percent . . . . ” However, the noise in that fiber laser output is solely noise of IR. By contrast, the noise in frequency converted light has additional (in some embodiments dominating) components, caused by mode interaction during conversion. Not all mechanisms of this excess noise are even clear yet.
Accordingly, an optical system including a polarization sensitive device that is not subject to output power fluctuation, and interference noise remains highly desirable.